Auto merge of #115025 - ouz-a:ouz_testing, r=lcnr

Make subtyping explicit in MIR

This adds new mir-opt that pushes new `ProjectionElem` called `ProjectionElem::Subtype(T)` to `Rvalue` of a subtyped assignment so we can unsoundness issues like #107205

Addresses #112651

r? `@lcnr`

Author: bors

Diff for: src/base.rs

@@ -876,6 +876,7 @@ pub(crate) fn codegen_place<'tcx>(
                 cplace = cplace.place_deref(fx);
             }
             PlaceElem::OpaqueCast(ty) => bug!("encountered OpaqueCast({ty}) in codegen"),
+            PlaceElem::Subtype(ty) => cplace = cplace.place_transmute_type(fx, fx.monomorphize(ty)),
             PlaceElem::Field(field, _ty) => {
                 cplace = cplace.place_field(fx, field);
             }

Diff for: src/value_and_place.rs

@@ -674,6 +674,16 @@ impl<'tcx> CPlace<'tcx> {
         }
     }
 
+    /// Used for `ProjectionElem::Subtype`, `ty` has to be monomorphized before
+    /// passed on.
+    pub(crate) fn place_transmute_type(
+        self,
+        fx: &mut FunctionCx<'_, '_, 'tcx>,
+        ty: Ty<'tcx>,
+    ) -> CPlace<'tcx> {
+        CPlace { inner: self.inner, layout: fx.layout_of(ty) }
+    }
+
     pub(crate) fn place_field(
         self,
         fx: &mut FunctionCx<'_, '_, 'tcx>,